Neither lifted a weapon to stop him. They hoped their appeal to Mike's humanity might have stopped him, and it almost did. Of course, it could be argued that what Mike did was in humanity's best interests, given Lily's propensity for dealing out death. (I'm assuming here that she's indeed dead - you clearly never know for sure on The Following - and short of someone's head being blown clean off, you never know for sure on any television show.)
But was Mike's pulling the trigger multiple times - which he certainly did, even if Lily somehow survives - in his best interests? Ryan thinks not, and sees what Mike did as turning Mike into Ryan, which for Ryan in a Shakespearean moment is a bad thing. More practically, what will Max do about having witnessed this execution? I can't see her arresting him or turning him in to the authorities - but I can't see her just forgetting about it, either.
As for the series, as I said in my review of last week's episode, I was liking Lily as a killing-cult leader better than Joe, so she'll be missed. But Joe became a little more interesting last night, in the intense conversation with Ryan in which Joe indicated that he knew he didn't have many more days left on this Earth. I had thought, earlier in the season, that it might end with Joe being killed and Lily surviving. But with Lily now presumably dead, will this season end with Joe being killed - for real, this time - too?
One point to bear in mind is that Joe was at his fatalistic worst before he learned that Claire was still alive. Might that give him more of a will to live?
Should be a compelling season finale next week.
And see also The Following Begins ... The Following 1.2: Joe, Poe, and the Plan ... The Following 1.3: Bug in the Sun ... The Following 1.4: Off the Leash ... The Following 1.5: The Lawyer and the Swap ... The Following 1.7: At Large ... The Following 1.9: All in a Name, Or, Metaphor in the Service of Murder ... The Following 1.13: At Last Something of a Day for the Good Guys ... The Following Season 1 Finale: Doing Dead
Like a Neanderthal serial killer in the current world? Try The Silk Code